Impact absorbing material

ABSTRACT

An impact absorbing material formed by: casting an aluminum alloy into a billet, the aluminum alloy including 5.6 to 6.6 wt % of zinc, 0.75 to 1.10 wt % of magnesium, 0.10 to 0.25 wt % of copper, 0.05 to 0.20 wt % of manganese, 0.03 to 0.15 wt % of chromium, 0.10 to 0.25 wt % of zirconium, 0.02 to 0.10 wt % of silicon, 0.05 to 0.17 wt % of iron, and the balance being aluminum and unavoidable impurities; extruding the billet to obtain an extruded shape having a hollow cross-section; and subjecting the extruded shape to artificial aging so that 0.2% proof stress is in a range of 340 to 385 MPa.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No. PCT/JP03/13588, having an international filing date of Oct. 23, 2003, which designated the United States, the entirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an impact absorbing material such as an automobile bumper reinforcement or impact beam. More particularly, the present invention relates to an impact energy absorbing member which absorbs impact energy at the time of collision or the like and is formed by using an aluminum alloy extruded shape.

It has been widely known that an extruded shape using Aluminum-Zinc-Magnesium alloys exhibits high strength.

In an automobile impact absorbing material such as an automobile bumper reinforcement or impact beam, an increase in strength contributes to a reduction in the weight of parts, thereby reducing the weight of automobiles. This contributes to a reduction in fuel consumption.

However, conventional Al—Zn—Mg alloys exhibit high tensile strength, but has inferior toughness. The alloys not only decrease the amount of impact energy absorption, but also result in occurrence of breakage or cracks at the time of collision (impact).

If such an extruded shape is used for a bumper reinforcement or the like, the edges of cracks occurring at the time of collision may cause injury, thereby posing a serious safety problem.

Japanese Patent Application Laid-open No. 2002-327229 has proposed an aluminum alloy extruded product having excellent crushing characteristics, for example.

However, the aluminum alloy extruded product disclosed in Japanese Patent Application Laid-open No. 2002-327229 easily produces cracks at the time of collision and has insufficient impact energy absorption. Moreover, this aluminum alloy extruded product has insufficient stress corrosion cracking resistance.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided an impact absorbing material formed by:

-   -   casting an aluminum alloy which includes 5.6 to 6.6 wt % of zinc         (Zn), 0.75 to 1.10 wt % of magnesium (Mg), 0.10 to 0.25 wt % of         copper (Cu), 0.05 to 0.20 wt % of manganese (Mn), 0.03 to 0.15         wt % of chromium (Cr), 0.10 to 0.25 wt % of zirconium (Zr), 0.02         to 0.10 wt % of silicon (Si), and 0.05 to 0.17 wt % of iron         (Fe), the balance being aluminum and unavoidable impurities, to         obtain a billet;     -   extruding the billet to obtain an extruded shape having a hollow         cross-section; and     -   subjecting the extruded shape to artificial aging so that 0.2%         proof stress is in a range of 340 to 385 MPa.

According to another aspect of the present invention, there is provided an automobile impact absorbing material comprising the above-described impact absorbing material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a table showing components of aluminum alloys used in examination according to one embodiment of the present invention.

FIG. 2 is a table showing characteristics of extruded shapes formed of aluminum alloys including the components shown in FIG. 1.

FIG. 3 is a graph showing energy absorption of a specimen having a hollow cross-section with a predetermined size when applying load until cracks occur.

FIG. 4 is a graph showing effects of Fe and Si on energy absorption.

DETAILED DESCRIPTION OF THE EMBODIMENT

In Al—Zn—Mg alloys, iron and silicon have been treated as impurities.

The present inventor has examined the relationship between proof stress and energy absorption in detail. As a result, the present inventor has found that high energy absorption can be stably obtained by controlling the Fe content and the Si content in a predetermined range.

The energy absorption changes to a large extent depending on artificial aging conditions merely by controlling the content of the aluminum alloy components in a predetermined range. This makes it difficult to stably prevent occurrence of cracks at the time of collision.

According to one embodiment of the present invention, high energy absorption can be stably obtained by controlling the artificial aging conditions so that 0.2% proof stress is in the range of 320 to 390 MPa.

Stress corrosion cracks generally proceed from the extrusion surface of the extruded shape. The present inventor has found that stress corrosion cracking resistance is improved by controlling the shape temperature during extrusion in the range of 500 to 540° C.

Specific details of the embodiment is further described below.

As aluminum alloy components suitable for an impact absorbing material formed by an aluminum alloy extruded shape according to this embodiment, the Zn content is controlled in the range of 5.6 to 6.6 wt % (hereinafter simply denoted by “%”), the Mg content is controlled in the range of 0.75 to 1.10%, the Cu content is controlled in the range of 0.10 to 0.25%, the Mn content is controlled in the range of 0.05 to 0.20%, the Cr content is controlled in the range of 0.03 to 0.15%, the Zr content is controlled in the range of 0.10 to 0.25%, the content of Fe which has been treated as an impurity is controlled in the range of 0.05 to 0.17%, the content of Si which has been treated as an impurity is controlled in the range of 0.02 to 0.10%, and the balance substantially consists of Al.

1. Zinc (Zn)

Zn affects aging characteristics to a large extent together with Mg. Since the atomic radius of Zn is comparatively close to the atomic radius of Al, a change in deformation resistance during extrusion is small. Therefore, in order to control proof stress in a predetermined range while reducing the Mg content, the Zn content is preferably controlled in the range of 5.6 to 6.6%.

If the Zn content is less than 5.6%, proof stress is decreased. If the Mg content is increased to compensate for a decrease in the Zn content, extrudability is decreased. In particular, in the case of forming an extruded shape having a hollow cross section, the extrusion speed is considerably decreased.

If the Zn content exceeds 6.6%, stress corrosion cracking resistance becomes poor.

2. Magnesium (Mg)

Mg affects aging characteristics to a large extent together with Zn.

Therefore, in order to control proof stress in a predetermined range, the Mg content is preferably controlled in the range of 0.75 to 1.10%.

Mg has an atomic radius greater than the atomic radius of Al, and significantly affects deformation resistance during extrusion. If the Mg content exceeds 1.10%, extrudability becomes poor. If the Mg content is less than 0.75%, proof stress is decreased when the Zn content is controlled in the range of 5.6 to 6.6%.

3. Copper (Cu)

Cu reduces the potential difference between the grain boundary and inside the crystal grain of the aluminum alloy to improve stress cracking resistance. If the Cu content is less than 0.10%, the effect is insufficient. If the Cu content exceeds 0.25%, general corrosion resistance is decreased. Therefore, the Cu content is preferably controlled in the range of 0.10 to 0.25%.

4. Zirconium (Zr), Manganese (Mn), and Chromium (Cr)

Zr affects the fiber structure to a large extent. If the Zr content is 0.10% or less, the effect is insufficient. If the Zr content exceeds 0.25%, the molten metal temperature must be increased during billet casting, thereby resulting in a decrease in castability.

Therefore, the Zr content is preferably controlled in the range of 0.10 to 0.25%.

Mn refines the grain size.

If the Mn content is less than 0.05%, the effect is insufficient. If the Mn content exceeds 0.20%, extrudability becomes poor and quench sensitivity is excessively increased. Therefore, Cr is added while limiting the Mn content to 0.20% or less.

Cr refines the crystal grains in the same manner as Mn, and causes the metal structure to become a fiber structure together with Zr.

However, Cr segregates as the amount of addition is increased. Therefore, the Cr content is preferably controlled in the range of 0.03 to 0.15%.

5. Iron (Fe)

Fe, which is the characteristic component in this embodiment, is mixed as an impurity in a casting step of an aluminum alloy billet.

Therefore, the Fe content is controlled in the range of 0.05 to 0.17% by preventing Fe from being mixed in a melting furnace and through a casting path.

The research conducted by the present inventor has revealed that Fe considerably affects toughness.

The effect of Fe on toughness drastically changes at an Fe content of 0.17 to 0.40%. Stable mechanical properties and energy absorption cannot be obtained unless the Fe content is limited to 0.17% or less. The Fe content is preferably 0.15% or less. In the case of forming a product such as a bumper reinforcement for which particularly stable impact energy absorption is required, the Fe content is ideally limited to 0.10% or less.

6. Silicon (Si)

Si also significantly affects toughness of the aluminum alloy extruded shape. In order to improve toughness while controlling proof stress in a predetermined range, the Si content is preferably controlled in the range of 0.02 to 0.10%. The Si content is ideally 0.06% or less in order to further stabilize mechanical properties and toughness and to secure predetermined energy absorption.

A billet is cast using an aluminum alloy in which the range of each component is controlled as described above and the balance substantially consists of aluminum.

In this case, Fe and Si are prevented from being mixed as much as possible during dissolution of a base metal, added master alloy, or the like and during pouring into a billet continuous casting die.

The resulting aluminum alloy billet is homogenized at 440 to 500° C. for four hours or more and extrusion-pressed to form an extruded shape having a hollow cross section.

The aluminum alloy in this embodiment excels in extrudability, and produces an thin extruded shape having a hollow cross section.

Automobile parts such as a bumper reinforcement are generally subjected to a post-treatment such as press bending corresponding to the body shape or the like.

Therefore, it is important to take measures to secure stress corrosion cracking resistance by controlling the extrusion conditions.

Conventionally, billet heating conditions, extrusion die temperature, extrusion speed, and the like are set taking the shape and surface quality of the resulting extruded shape into consideration.

The present inventor has revealed that the temperature of the shape immediately after extrusion (near the area extruded from the extrusion die) considerably affects stress corrosion cracking resistance, and has found that it is preferable to control the temperature of the extruded shape during extrusion in the range of 500 to 540° C.

If the shape temperature exceeds 540° C., the size of recrystallized grains is increased, whereby stress corrosion cracking resistance is rapidly decreased.

If the shape temperature is less than 500° C., extrusion productivity is decreased.

Artificial aging of the extruded shape after extrusion is important in order to secure stable and high impact energy absorption.

The extruded shape is subjected to two-stage artificial aging at 70 to 170° C. for 6 to 20 hours.

For example, a first-stage heat treatment is performed at 70 to 95° C. for 3 to 8 hours, and a second-stage heat treatment is performed at 130 to 170° C. for 3 to 12 hours.

The objective of this artificial aging is to prevent occurrence of cracks at the time of collision by refining and uniformly dispersing precipitates to increase toughness.

Therefore, it is preferable to perform the artificial aging at a comparatively low temperature for a long period of time. The artificial aging conditions are set so that 0.2% proof stress is in the range of 320 to 390 MPa.

Since the heat treatment conditions differ depending on the hollow cross section and the thickness of the shape, it is preferable to examine and set the heat treatment conditions in advance using a specimen.

FIG. 1 shows components of aluminum alloys used to examine the effects of this embodiment.

Examples 1 to 5 are alloys according to this embodiment, and Comparative Examples 1 to 4 are alloys for confirming the effects of the components.

The alloys of Examples 1 to 3 are designed in order to confirm the effects of Fe and Si, in which the content of other components is approximately the same.

The alloy of Example 4 is designed aiming at the upper limit of each component, and the alloy of Example 5 is designed aiming at the lower limit.

The alloy of Comparative Example 1 is designed so that the content of each component exceeds the upper limit of this embodiment, and the alloy of Comparative Example 2 is designed so that the content of the components other than Fe and Si is less than the lower limit.

The alloys of Comparative Examples 3 and 4 are designed so that the content of Fe and Si is greater than the upper limit of this embodiment.

8-inch billets were manufactured using these aluminum alloys. The billets were homogenized at 480° C. for 12 hours, and extrusion-pressed to obtain extruded shapes. FIG. 2 shows examination results for quality characteristics of the extruded shapes.

The extruded shapes had a triple hollow cross section in a shape of a Chinese character meaning an eye (which is like a hollow quadrangle with two stays at the inside thereof) with outer dimensions of about 65 mm×125 mm and a thickness of 2 to 4 mm.

Among the evaluation items shown in FIG. 2, mechanical properties were measured for a Japanese Industrial Standard No. 5 specimen cut from the extruded shape.

The EA amount is the amount of energy absorption when applying load. The extruded shape having a triple hollow cross section in a shape of a Chinese character meaning an eye (which is like a hollow quadrangle with two stays at the inside thereof) was cut to a length of 300 mm, and load was applied to the plane rigid surface. The amount of energy absorption was measured from the area of the load-stroke diagram.

SCC is the result of a stress corrosion cracking resistance test, in which occurrence of cracks was evaluated after immersing the specimen to which a predetermined bending stress was applied in the following test liquid.

Test liquid: chromic oxide: 36 g/L, potassium dichromate: 30 g/L, sodium chloride: 3 g/L, 50° C.

In the table shown in FIG. 2, “72, YES” means that no cracks occurred after 72 hours of immersion, and “22, NO” means that cracks occurred after 22 hours of immersion.

The “speed” means the extrusion speed when extruding an extruded shape having a triple hollow cross section in a shape of a Chinese character meaning an eye (which is like a hollow quadrangle with two stays at the inside thereof). For example, “6, YES” means that the shape could be extruded at a speed of 6 m/min, and “2, NO” means that the shape could be extruded at a speed of 2 m/min, but resulted in poor productivity.

The shape temperature shows a value obtained by measuring the surface temperature of the shape immediately after extrusion.

The BLT temperature means the preheating temperature of the billet.

As a result, Examples 1 to 5 satisfied the target quality, and Comparative Examples 1 to 4 did not satisfy the target quality for at least one of the items.

FIG. 3 is a graph showing the analysis results for the relationship between the proof stress and the amount of energy absorption based on the measurement results shown in FIG. 2.

The amount of energy absorption is generally decreased as the proof stress is decreased. However, this embodiment has revealed that the amount of energy absorption is rapidly decreased when the proof stress exceeds about 390 MPa.

This is because a considerable number of cracks occurred in the extruded shape in the process of increasing the load. Therefore, in order to secure a stable and large amount of impact energy absorption, it is preferable to control the 0.2% proof stress in the range of 320 to 390 MPa.

A graph shown in FIG. 4 shows the analysis results for the relationship between the proof stress and the amount of energy absorption with respect to the content of Fe and Si.

The plot showing a proof stress of 350 MPa or more corresponds to Examples 1, 2, and 3, and the plot showing a proof stress of 320 MPa or less is the value obtained from Comparative Examples 3 and 4.

This also reveals that Fe and Si affect the proof stress, and cause the amount of energy absorption to become unstable.

As a result of analysis of the relationship between SCC (stress corrosion cracking resistance) and the shape temperature from the results shown in FIG. 2, it is preferable to limit the shape temperature to 500 to 540° C.

A high and stable toughness (energy absorption characteristics accompanying cross-sectional deformation) is obtained by using the aluminum alloy shape according to this embodiment.

Therefore, the aluminum alloy shape according to this embodiment can be applied to a member which absorbs impact energy at the time of collision, such as a bumper reinforcement attached to the front and back of automobiles or an impact beam (side beam) attached inside the doors of automobiles.

In particular, a bumper reinforcement must be designed so as to absorb a predetermined amount of impact energy so that an airbag or the like is actuated when an excessive impact is applied.

In such an application field, it is difficult to design the product if the amount of energy absorption is unstable. However, since the amount of energy absorption can be accurately determined in advance by using the impact absorbing material according to this embodiment, the actuation conditions for the airbag or the like can be easily set. 

1. An impact absorbing material formed by: casting an aluminum alloy into a billet, the aluminum alloy including 5.6 to 6.6 wt % of zinc, 0.75 to 1.10 wt % of magnesium, 0.10 to 0.25 wt % of copper, 0.05 to 0.20 wt % of manganese, 0.03 to 0.15 wt % of chromium, 0.10 to 0.25 wt % of zirconium, 0.02 to 0.10 wt % of silicon, 0.05 to 0.17 wt % of iron, and the balance being aluminum and unavoidable impurities; extruding the billet to obtain an extruded shape having a hollow cross-section; and subjecting the extruded shape to artificial aging so that 0.2% proof stress is in a range of 340 to 385 MPa.
 2. The impact absorbing material as defined in claim 1, wherein the temperature of the extruded shape in extrusion is controlled to be in a range of 500 to 540° C. to improve the stress corrosion cracking resistance.
 3. An automobile impact absorbing material comprising the impact absorbing material as defined in claim
 1. 4. An automobile impact absorbing material comprising the impact absorbing material as defined in claim
 2. 